In recent years, there is an increasing demand for transparent plastic or resin film for optical applications, especially for high-quality protection film of polarizing plates for liquid crystal display devices. For the manufacture of such a plastic film, an apparatus using a solution film-forming method is suitable.
The solution film-forming method uses a solution called dope, which is made by dissolving material flakes with a solvent and adding some additive agents according to the needs, such as plasticizers, UV-absorbers, antidegradants, slipping agents, and a release accelerator. The solution film-forming method includes the steps of casting the dope from a dope supply device, called a casting die, on a support that is constituted of a horizontal endless metal band or a rotary drum, drying the dope on the support to some extent till it gets rigid to be a solidified film, peeling the solidified film off the support, and removing the solvent from the film while conveying it through a drying section.
On the other hand, in order to develop industrial production of plastic films, it is important to improve their productivity. To improve productivity, it is necessary to automatize as many manufacturing processes as possible. Especially for embodying a solution film-forming method of manufacturing plastic film, there is a large need for automatizing a casting apparatus, also because it will prevent discharging organic solvent into the environment and polluting the environment. Such an automatic casting apparatus is disclosed for example in Japanese Laid-open Patent Application No. Hei 8-244053.
However, the solution film-forming method of manufacturing plastic film or resin film can suffer from a trouble that is caused by the air contained in the dope. That is, when the dope is initially fed to the casting die at the start of operation, the air collects in the leading stream of the dope, and often forms bubbles. If the dope containing the bubbles is cast on the support for the film-forming, the resin solution or dope will splash and stick to die lips. This will result in forming undesirable streaks on the products.
To solve this problem, the present applicant has invented an apparatus for feeding the dope free from the air to the casting die, and filed a Patent application for this apparatus, as Japanese Laid-open Patent Application No. 2000-317961. This apparatus is provided with a second pipe or a dope tapping pipe in addition to a dope supply pipe for a casting die, in the vicinity of a dope entrance of the casting die. The flow of the dope may be switched over between the dope supply pipe and the dope tapping pipe, so that an initial flow of the dope, as containing the air, is conducted into the dope tapping pipe. Thereafter, the air-free dope is conduced through the dope supply pipe to the casting die.
However, it has been found that the apparatus disclosed in Japanese Laid-open Patent Application No. 2000-317961 cannot completely vent the air that collects in the dope supply pipe, so that it is difficult to automatize the starting process of the solution film-forming.